1. Field of the Invention
The present invention relates to a solid-state imaging device and particularly to a charge coupled device (CCD) type solid-state imaging device.
2. Description of the Related Art
FIGS. 1 and 2 are diagrams showing an arrangement of a CCD solid-state imaging device and particularly showing an arrangement of a unit cell portion thereof.
As shown in FIG. 1, a CCD solid-state imaging device 1 has a plurality of sensor portions 2 arranged in a matrix fashion, and vertical shift registers 3 having a CCD structure provided on each side of a series of the sensor portions.
A transfer electrode 4 of the vertical shift register 3 has a first electrode 5 formed of a first-layer polysilicon and a second electrode 6 formed of a second-layer polysilicon, and the first electrodes 5 and the second electrodes 6 of the transfer electrodes are repeatedly arranged along the vertical direction so as to correspond to the respective sensor portions 2. The first electrode 5 and the second electrode 6 are commonly formed with respect to each of the vertical shift registers 3.
A read gate portion 7 employing the second electrode 6, for example, as a gate electrode, i.e., a read gate portion for reading a signal charge from the sensor portion 2 to the vertical shift register 3 is formed between the sensor portion 2 and the vertical shift register 3 corresponding thereto. A channel stopper region 8 is formed between the sensor portion 2 and the vertical shift register 3 formed on the opposite side of the vertical shift register 3 adjacent to the read gate portion 7.
FIG. 2 is a cross-sectional view showing a semiconductor structure of the CCD solid-state imaging device 1 shown in FIG. 1 and cut along a line A--A shown in FIG. 1. In this semiconductor structure, a second conductive type, e.g., p-type first well region 12 is formed on a first conductive type, e.g., n-type silicone substrate 11, and the sensor portion 2, an n-type transfer region 13 forming the vertical shift register 3 and the p-type channel stopper region 8 are formed in the first p-type well region 12.
The sensor portion 2 is formed as a so-called hole accumulated sensor having the p-type well region 12, an n-type impurity region 15 formed therein, a charge accumulation layer formed on a surface of the n-type impurity region 15 and a p-type high-density impurity region 16 serving as a hole accumulation layer in this example.
A p.sup.- region 17 is formed in the read gate portion 7. A second p-type well region 18 is formed under the n-type transfer region 13.
The transfer electrode 4 made of polysilicon is formed through a gate insulating film 19 on the n-type transfer region 13 and the p.sup.- region 17 of the read gate portion 7. An interlayer insulating film 20 is formed on an entire surface including a surface of the transfer electrode 4, and further a light shielding film 21 made of, for example, Al or the like is formed on an entire surface other than at an opening portion 22 provided immediately over the sensor portion 2.
In the CCD type solid-state imaging device 1, when a voltage is applied to the second electrode 6 of the transfer electrode 4, signal charges accumulated in the sensor portion 2 by photoelectric conversion are read out therefrom through the read gate portion 7 to the vertical shift register 3 as shown by an arrow a in FIG. 1. Further, the transferred signal charges are shifted in the vertical direction in the vertical shift register 3 by a driving operation of the transfer electrode 4.
In order to suppress smear caused in the CCD solid-state imaging device by rays of light which are made incident from an opening-portion side edge portion of the light shielding film 21 and repeatedly reflected between the light shielding film 21 and the silicon substrate 11 to consequently reach the vertical shift register 3, a portion 21a (hereinafter referred to as a projection portion) from an edge portion, on the side of the sensor portion 2 of the transfer electrode 4, of the light shielding film 21 to an opening portion 22 thereof is formed so as to be long.
However, since in the CCD solid-state imaging device 1 shown in FIGS. 1 and 2 a width of the sensor portion 2 is set to an interval between the transfer electrodes 4, 4 adjacent to each other in the traverse direction of the sensor portion 2, miniaturization of a unit cell prevents the projection portion 21a of the light shielding film 21 from being made longer because a proper area of the opening portion 22 must be kept.
It can be considered to set the width of the transfer electrode 4 located immediately over the vertical shift register 3 narrower in order to project the projection portion 21a of the light shielding film 21 further. However, since the read gate portion 8 is provided between the sensor portion 2 and the vertical shift register 3 as described above, when the sensor portion 2 is not formed by using self-alignment relative to the transfer electrode 4, a barrier is generated between the sensor portion 2 and the read gate portion 8 and consequently prevents the signal charge from being read out from the sensor portion 2.
On the other hand, when the sensor portion 2 is formed by using the self-alignment with respect to the transfer electrode 4, the area of the read gate portion 8 becomes narrower, which makes the signal charges from the sensor portion 2 overflow in the vertical shift register when the signal charge is not read out and hence causes a so-called blooming.
Therefore, it is difficult to reduce smear further in the CCD solid-state imaging device 1. Smear tends to increase as the unit cell is more miniaturized.
Moreover, since the width of the sensor portion 2 is set equal to an interval between the transfer electrodes 4 adjacent to each other across the sensor portion 2 in the CCD solid-state imaging device 1, further miniaturization of the unit cell deteriorates a read characteristic and increases blooming. As a result, it is difficult to set the width of the read gate portion 7 narrower, which leads to deterioration of a device characteristic and increase of smear.
Therefore, it is difficult to form the opening portion 22 of the light shielding film 21, and the unit cell cannot be miniaturized beyond about 5 .mu.m.sup.2.
Moreover, when the signal charges in the sensor portion 2 are read out therefrom to a portion of the corresponding vertical shift register 3 under a predetermined transfer electrode, it is necessary to provide a channel stopper region 8 which should be minimum. In the arrangement shown in FIG. 2, in order to prevent the signal charges from leaking from the adjacent sensor portion 2 in the vertical direction, it is necessary to provide the read gate portion 7 at the second electrode 6 formed of the second-layer polysilicon.
Also, in this case, in order to suppress smear caused in the CCD solid-state imaging device by rays of light which are made incident from an opening-portion side edge portion of the light shielding film 21 and repeatedly reflected between the light shielding film 21 and the silicon substrate 11 to consequently reach the vertical shift register 3, a portion 21a (hereinafter referred to as a projection portion) from an edge portion on the side of the sensor portion 2 of the transfer electrode 4 of the light shielding film 21 to an opening portion 22 thereof is formed so as to be long.
Therefore, even if the unit cell is further miniaturized, it is impossible to reduce the width of the channel stopper region 8, which consequently leads to deterioration of the device characteristics and an increase of the smear.